The present invention relates to a method for winding a fibre element having different longitudinal portions. In particular, the present invention relates to a method for winding an optical fibre having different longitudinal portions onto a reel at the end of a process for drawing the said fibre.
For the purposes of the present invention, xe2x80x9cfibre elementxe2x80x9d is understood as meaning an optical fibre, where appropriate provided with a surface coating and with other coatings.
As is known in the art, optical fibres are produced in special drawing towers from preforms which have been prepared beforehand. In practice, a preform is supplied, along a vertical direction, to a furnace so as to obtain a casting of molten material. The molten material is then drawn and cooled so as to obtain an optical fibre with the desired characteristics. These characteristics are obtained by suitably setting the parameters of the drawing process, namely the furnace temperature, the fibre drawing speed, and all the other parameters (described in detail below) which define the process conditions. At the end of the drawing process, the optical fibre is wound onto a storage reel, from where it is later unwound at the time of use, either for carrying out tests or so as to rewind it onto other reels.
Generally, the drawing process is carried out for its entire duration under the same process conditions, i.e. without any variation in the process parameters. In such a case, a fibre is drawn from a single preform such that it is substantially homogeneous and uniform along its whole length. At the end of the production process, this fibre is wound onto the storage reel in a cylindrical helix, without interruption and with a constant pitch. For the purposes of the present invention, xe2x80x9cwinding pitchxe2x80x9d is understood as meaning the distance between two consecutive intersections of the helix with the same generatrix of the cylinder on which the said helix lies. Helical winding with a constant pitch is usually obtained by moving the reel axially so that it performs an alternating movement at a constant speed or by moving a member which supplies the fibre to the reel so that it performs a similar movement.
Drawing towers of the known type, owing to recent progress in technology, allow optical fibres which are up to several hundred kilometers long to be obtained from a single preform. In the future, technological improvements will probably result in even longer fibres being obtained from a single preform. In view of the high yields which can already be obtained nowadays in drawing processes, it may be necessary or advantageous to form, from a single preform, a fibre comprising longitudinal portions which have different chemical/physical characteristics, for example longitudinal portions with different core and/or cladding diameters, different internal tensions, etc. These physical variations may be obtained by changing, during the fibre drawing process, one or more characteristic process parameters.
For example, it may be advantageous to form a fibre comprising two or more longitudinal portions having substantially the same basic characteristics, but with one or more differences of an optical, geometric or mechanical nature. A first example is provided by M. Ohashi et al. in xe2x80x9cDispersion-Modified Single Mode Fiber by VAD Methodxe2x80x9d, NTT, Japan, The Transaction of the IEICE, Vol. E 73, No. 4, April 1990. In this article, in order to study the sensitivity of single mode optical fibres with a low dispersion of about 1.5-1.6 xcexcm to micro-bendings, fibre portions which are several kilometers long are used, said fibres being made from a single preform and being different from one another solely in terms of the zero-chromatic-dispersion wavelength.
A further example relates to the study of the phenomenon of modal dispersion due to polarization (xe2x80x9cPolarization Mode Dispersionxe2x80x9dor PMD). As is known, this phenomenon is influenced by the photoelastic effect which occurs in the fibre, this being dependent upon structural characteristics associated with the tension to which the said fibre is subjected during the drawing process. For this type of study it is therefore advantageous to have fibre portions available that have the same physical/chemical characteristics and different internal tensions. U.S. Pat. No. 5,400,422 proposes a technique for forming, in an optical fibre, a sequence of Bragg gratings of varying pitch, during the actual fibre drawing process. The gratings are formed by exposing the fibre, using optical techniques of the interferometric type, to ultraviolet radiation pulses. The gratings are formed at predefined distance from each other along the fibre. In this case also, therefore, fibre portions with different characteristics are formed by varying, during the drawing process, the operating conditions of the process.
In other instances it may be necessary to produce local variations in the characteristics of the drawn fibre, i.e. variations affecting only small sections of the said fibre. A situation of this type is described in U.S. Pat. No. 4,163,370, in which a short section of fibre with a larger diameter is formed in order to improve the performance of the fibre in terms of modal dispersion.
The Applicant has noticed that, in cases where it is required to form a fibre comprising longitudinal portions which are different from one another, winding onto the reel without interruptions and with a constant pitch is not very advantageous since the different portions of wound fibre cannot subsequently be distinguished from one another. One way of solving this problem is to interrupt the winding process and, therefore, the drawing process, whenever a given fibre portion has been completely wound so as to be able to cut the fibre at the end of this portion and replace the reel onto which this portion has been wound with an empty reel. At this point, the winding process (and the drawing process) can be started again and the next fibre portion is wound onto a new reel. The need to interrupt the drawing process every time, however, gives rise to certain drawbacks, including loss of time, wastage of molten material from the preform and a possible variation in the drawing conditions, for example due to temporal variations in the characteristic parameters of the process.
Alternatively, if the fibre must be completely wound onto the same reel, it is possible, instead of cutting the fibre at the end of each wound portion, apply a marker to a predefined point on each portion, which can be subsequently identified. This operation may even be performed without interrupting the drawing process, as described in the already cited U.S. Pat. No. 5,400,422. However, the use of markers to identify the different fibre portions is unreliable since, at the normal fibre unwinding speeds, one or more of these markers could be accidentally overlooked. Moreover, this technique generally requires the presence of an operator during unwinding of the fibre, who is responsible for identification of the markers.
The Applicant has found that, in the case of a fibre comprising longitudinal portions which are different from one another, if the fibre is wound by associating with each portion a respective winding pitch different from the winding pitches of the adjacent portions, subsequent identification of these portions during unwinding of the fibre can be performed rapidly, automatically and with very small probability of error. The Applicant proposes, inter alia, a winding method which can be performed without interruptions, thus avoiding the abovementioned drawbacks.
The Applicant has also found that, in order to reduce to a minimum the probability of error in identifying the various fibre portions during unwinding of the said fibre, it is preferable to modulate the winding pitch with periodic functions. This variation in the winding pitch is preferably performed by modulating, with the same law, the speed of axial translation of the reel during winding. If the fibre has been wound in this way, subsequent identification of the different fibre portions during unwinding from the reel is performed, in accordance with the invention, by detecting the variations in the winding pitch.
According to a first aspect, the present invention relates to a method for winding a fibre element onto a support, said fibre element comprising at least two longitudinal portions with different characteristics, said method comprising the step of supplying said fibre element to said support and being characterized in that it comprises the step of associating with each of said portions a respective value of a winding parameter which is different from the values associated with the portions adjacent thereto. In particular, said step of associating a respective value of a winding parameter with each of said portions comprises the step of associating a respective winding pitch with each of said portions.
In particular, said step of associating a respective winding pitch with each of said portions comprises the step of associating a respective function for modulation of said winding pitch with each of said portions.
Preferably said step of associating a respective function for modulation of said winding pitch with each of said portions comprises the step of associating a respective frequency for modulation of said winding pitch with each of said portions, said modulation frequency defining the main frequency of a respective periodic modulation function.
In particular, said step of associating a respective winding pitch with each of said portions comprises the step, performed at the same time as said step of supplying said fibre element to said support, of translating said support in a predefined direction at a speed correlated to said winding pitch.
Said step of supplying said fibre element to said support comprises the step of directing said fibre element towards said support by means of a supply member and, by way of an alternative to that described above, said step of associating a respective winding pitch with each of said portions comprises the step, performed at the same time as said directing step, of translating said supply member in a predefined direction at a speed correlated to said winding pitch.
Preferably said step of supplying said fibre element to said support is associated with a starting instant and with an end instant and comprises the steps of measuring the time between said starting instant and said end instant and to associate with each of said portions a respective time interval between said starting instant and said end instant.
Preferably, said fibre element is an optical fibre, said step of supplying said fibre element to said support being performed at the same time as a step for producing said optical fibre; said production step comprising the step of drawing said optical fibre from a preform. Said production step comprises the preliminary step of setting process parameters so as to obtain predefined characteristics of said fibre element, each of said portions being associated with a respective set of values of said parameters.
Moreover, preferably, said production step comprises the step, performed during said drawing step, of measuring process variables and signalling, if one of said variables exceeds a predefined threshold, a respective alarm condition indicating the presence of a fibre portion with a defect; said step of associating a respective value of a winding parameter with each of said portions comprises the step of associating a respective value of said winding parameter with said fibre portion which has a defect.
According to a further aspect, the present invention relates to a method for distinguishing different longitudinal portions of a fibre element wound onto a support according to the method described above, each of said portions having associated with it a respective winding pitch, characterized in that it comprises the steps of:
unwinding said fibre element from said support;
detecting, during said unwinding step, variations in said winding pitch;
In particular, said step of detecting variations in said winding pitch comprises the steps of:
repeatedly measuring, during said unwinding step, a parameter correlated to said winding pitch in order to obtain successive values of said parameter; and
detecting variations in the value of said parameter.
Preferably, said method for distinguishing different longitudinal portions also comprises the steps of:
comparing each of said obtained values of said parameter with a set of stored values, each of said stored values being associated with one of said portions; and
identifying, on the basis of said comparison, the longitudinal portion associated with said value obtained.
Preferably, said step of detecting variations in the value of said parameter comprises the steps of:
storing a value obtained;
comparing a successive value of said parameter with said stored value; and
interrupting said unwinding step if, during said comparison step, said successive value is different from said stored value.
Preferably, said step of measuring said parameter comprises the step of detecting the distance between an actual point where a predefined area is crossed by the fibre element and a predefined crossing point of the said area.
By way of an alternative, said step of measuring said parameter comprises the step of detecting the angle between a direction of unwinding of the fibre element from the support and a predefined direction.
According to a further aspect, the present invention relates to an apparatus for winding a fibre element onto a support, said fibre element comprising at least two longitudinal portions with different characteristics, said apparatus comprising:
a supply member for supplying said fibre element to said support; and
a moving device for moving either one or other of said support and said supply member along a predefined axis and at a predefined speed of translation so as to obtain a predefined winding pitch;
and being characterized in that it comprises a unit for controlling said moving device, designed to control said speed of translation so as to associate with each of said portions a respective winding pitch different from the winding pitches associated with the portions adjacent thereto.
Preferably, at least one of said winding pitches is modulated with a periodic function.
Preferably, said fibre element is an optical fibre. According to further aspect, the present invention relates to an apparatus for distinguishing different longitudinal portions of a fibre element wound onto a support, each of said portions having associated with it a respective winding pitch, characterized in that it comprises:
a device for unwinding said fibre element from said support;
a sensor device designed to measure repeatedly a parameter correlated to said winding pitch and generate a signal indicating said parameter; and
a processing unit designed to receive said signal and detect, on the basis of said signal, variations in said parameter.
In particular, said processing unit comprises:
a comparison sub-unit for comparing successive values of said parameter; and
a signalling sub-unit for signalling the presence of a new portion if said successive values are different from one another.
Preferably, said sensor device is an optical device which has a sensitive area and a reference point on said sensitive area and is designed to detect the distance between the point where said sensitive area is crossed by the said fibre element and said reference point.
Alternatively, said sensor device is a device designed to detect the angle between a direction of unwinding of the fibre element from the support and a predefined direction.
According to a further aspect, the present invention relates to a method for producing a fibre element, said method comprising the step of drawing said fibre element from a preform and forming two longitudinal portions with different characteristics of said fibre element, and being characterized in that it comprises the step of winding said two longitudinal portions onto a support, associating a respective winding pitch with each of said two longitudinal portions.
According to a last aspect, the present invention relates to an assembly for the production of a fibre element, comprising a production apparatus designed to produce a fibre element comprising at least two longitudinal portions having characteristics which are different from one another; said assembly being characterized in that it also comprises a winding apparatus designed to receive said fibre element from said production apparatus and wind said fibre element onto a support, associating a respective winding pitch with each of said portions.
In particular, said winding apparatus comprises:
a supply member for supplying said fibre element to said support in a predefined supplying direction; and
an axial moving device for moving either one or other of said support and said supply member in a predefined direction and at an axial speed which, for each of said portions, is correlated to the winding pitch associated with said portion.
Preferably, said winding apparatus comprises a control unit connected to said axial moving device for controlling said axial speed.
Preferably, said production apparatus comprises sensor devices connected to said control unit, each of said sensor devices being designed to detect a respective process variable.
Preferably, said assembly also comprises an apparatus for distinguishing different longitudinal portions of said fibre element wound onto said support, said distinguishing apparatus comprising:
an unwinding device designed to unwind said fibre element from said support; and
a detector device for detecting variations in said winding pitch during unwinding of said fibre element.
In particular, said detector device comprises:
a sensor device designed to generate a signal correlated to said winding pitch;
a processing unit designed to receive said signal and obtain, from said signal, a value indicating said winding pitch.
In particular, said sensor device is an optical sensor designed to be crossed by said fibre element.
Preferably said fibre element is an optical fibre and said production apparatus is a drawing tower.